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Butyric acid, (from Greek βουτυρος = butter) IUPAC name n-Butanoic acid, or normal butyric acid, is a carboxylic acid with structural formula CH3CH2CH2-COOH. It is found in rancid butter, parmesan cheese, and vomit, and has an unpleasant odor and acrid taste, with a sweetish aftertaste (similar to ether). Butyric acid can be detected by mammals with good scent detection abilities (e.g., dogs) at 10 ppb, whereas humans can detect it in concentrations above 10 ppm.

Butyric acid is a fatty acid occurring in the form of esters in animal fats and plant oils. The glyceride of butyric acid makes up 3% to 4% of butter. When butter goes rancid, butyric acid is liberated from the glyceride by hydrolysis leading to the unpleasant odor. It is an important member of the fatty acid sub-group called short chain fatty acids.

Normal butyric acid or fermentation butyric acid is also found as a hexyl ester (hexyl butanoate) in the oil of Heracleum giganteum (cow parsnip) and as an octyl ester (octyl butanoate) in parsnip (Pastinaca sativa); it has also been noticed in the fluids of the flesh and in perspiration.

It is ordinarily prepared by the fermentation of sugar or starch, brought about by the addition of putrefying cheese, with calcium carbonate added to neutralize the acids formed in the process. The butyric fermentation of starch is aided by the direct addition of Bacillus subtilis.

Butyric acid is used in the preparation of various butyrate esters. Low-molecular-weight esters of butyric acid, such as methyl butyrate, have mostly pleasant aromas or tastes. As a consequence, they find use as food and perfume additives. They are also used in organic laboratory courses, to teach the Fisher esterification reaction.

The acid is an oily colorless liquid that solidifies at -8 °C; it boils at 164 °C. It is easily soluble in water, ethanol, and ether, and is thrown out of its aqueous solution by the addition of calcium chloride. Potassium dichromate and sulfuric acid (also known as sulphuric acid) oxidize it to carbon dioxide and acetic acid, while alkaline potassium permanganate oxidizes it to carbon dioxide. The calcium salt, Ca(C4H7O2)2·H2O, is less soluble in hot water than in cold.

There is an isomer, isobutyric acid, which has the same chemical formula C4H8 O2 but a different structure. It has similar chemical properties but different physical properties.

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Butyrate fermentation

Butyrate is produced as end-product of a fermentation process solely performed by obligate anaerobicbacteria. Kombucha tea includes butyric acid as a result of fermentation. This fermentation pathway was discovered by Louis Pasteur in 1861. Examples of butyrate-producing species :

The pathway starts with the glycolytic cleavage of glucose to two molecules of pyruvate, as happens in most organisms. Pyruvate is then oxidized into acetyl coenzyme A using a unique mechanism that involves an enzyme system called pyruvate-ferredoxin oxidoreductase. Two molecules of carbon dioxide (CO2) and two molecules of elemental hydrogen (H2) are formed in the process and exit the cell. Then:

ATP is produced, as can be seen, in the last step of the fermentation. Three molecules of ATP are produced for each glucose molecule, a relatively high yield. The balanced equation for this fermentation is:

C6H12O6 → C4H8O2 + 2CO2 + 2H2

Acetone and butanol fermentation

Several species form acetone and butanol in an alternative pathway, which starts as butyrate fermentation. Some of these species are:

These bacteria begin with butyrate fermentation as described above, but, when the pH drops below 5, they switch into butanol and acetone production in order to prevent further lowering of the pH. Two molecules of butanol are formed for each molecule of acetone.

The change in the pathway occurs after acetoacetyl CoA formation. This intermediate then takes two possible pathways:

Acetoacetyl CoA → acetoacetate → acetone, or

Acetoacetyl CoA → butyryl CoA → butyraldehyde → butanol.

Butyric acid function/activity

Highly-fermentable fibers like oat bran, pectin, and guar are transformed by colonic bacteria into short-chain fatty acids including butyrate.

Butyrate has diverse and, it seems, paradoxical effects on cellular proliferation, apoptosis and differentiation that may be either pro-neoplastic or anti-neoplastic, depending upon factors such as the level of exposure, availability of other metabolic substrate, and the intracellular milieu. Butyrate is thought by some to be protective against colon cancer. However, not all studies support a chemopreventive effect for butyrate, and the lack of agreement (particularly between in vivo and in vitro studies) on butyrate and colon cancer has been termed the "butyrate paradox." There are many reasons for this discrepant effect, including differences between the in vitro and in vivo environments, the timing of butyrate administration, the amount of butyrate administered, the source of butyrate (usually dietary fiber) as a potential confounder, and an interaction with dietary fat. Together, the studies suggest that the chemopreventive benefits of butyrate depend in part on amount, time of exposure with respect to the tumorigenic process, and the type of fat in the diet.[1] Low carbohydrate diets like the Atkins diet are known to reduce the amount of butyrate produced in the colon.

Butyric acid has been associated with the ability to inhibit the function of histone deacetylase enzymes, thereby favouring an acetylated state of histones in the cell. Acetylated histones have a lower affinity for DNA than non-acetylated histones, due to the neutralisation of electrostatic charge interections. In general, t is thought that transcription factors will be unable to access regions where histones are tightly associated with DNA (ie non-acetylated, e.g., heterochromatin). Therefore, it is thought that butyric acid enhances the transcriptional activity at promoters, which are typically silenced/downregulated due to histone deacetylase activity.